(a) Field of the Invention
The present invention relates to lifting mechanisms adapted to raise weights and, more particularly, to a three-shift lifting mechanism, which provides three lifting shifts, i.e., the first shift to lift the lifting tube rapidly when bearing no weights, the second shift to lift the lifting tube to raise the weights rapidly, and the third shift to buffer the lifting upon an overload at the lifting tube.
(b) Description of the Prior Art
A conventional lifting mechanism for use in a carriage jack or the like is generally comprised of a valve block, a hydraulic device, and a piston pump. The piston pump is driven by a crank to suck in hydraulic oil and then to pump hydraulic oil through oil passages in the valve block, causing the lifting tube of the hydraulic device to raise the weights. Because of simple oil passage design, this structure of lifting mechanism can only move the lifting rod at a predetermined speed, i.e., the lifting tube is lifted at a low speed either when the lifting tube bearing or not bearing weights. Due to this limitation, the user must employ much effort to repeatedly reciprocate the piston pump when lifting the lifting tube to the bottom side of the weights to be raised.
There are known dual-speed lifting mechanisms that enable the lifting tube to be moved to the bottom side of the weights at a high speed and then lifted to raise the weights at a low speed. However, these dual-speed lifting mechanism have no means to buffer the weights upon an overload. Upon an overload, the hydraulic device may be destroyed, or caused to leak oil. Further, the piston pump sucks in hydraulic oil only during its upstroke, i.e. the piston pump cannot simultaneously suck in hydraulic oil when pumping out hydraulic oil to lift the lifting tube.
Therefore, it is desirable to provide a three-shift lifting mechanism that eliminates the drawbacks of the conventional lifting mechanisms.